1. Field of the Invention
This invention relates to an uninterruptible power supply with a voltage step-up chopper, and a starting method thereof.
2. Description of the Prior Art
FIG. 1 is a block diagram showing one example of a conventional uninterruptible power supply. In FIG. 1, the reference numeral 100A designates an uninterruptible power supply which includes an AC input interrupter 2 connected to a commercial power source 1, a battery 3, a DC input interrupter 4, and a controlled rectifier 5 which includes thyristors and simultaneously operates as a rectifier and a charger for the battery 3. The uninterruptible power supply 100A also includes an inverter 6, an inverter transformer 7 which isolates the inverter 6 from a load (not shown) for insulation, and an AC filter 8 which improves the waveform of the secondary voltage of the inverter transformer 7 so as to be sinusoidal.
As a starting method for the uninterruptible power supply 100A shown in FIG. 1, usually both the AC input interrupter 2 and the DC input interrupter 4 are previously closed, and under this condition the controlled rectifier 5 is started up so as to gradually raise the output voltage thereof, then the inverter 6 is started up so as to gradually raise the output voltage of the uninterruptible power supply 100A by the pulse width control of the inverter 6.
In case that an increase in capacity of the uninterruptible power supply 100A is required, the capacity of semiconductor elements that constitute the inverter 6 would be insufficient. This then necessitates that the inverter 6 includes a plurality of semiconductor elements in parallel connection or that the inverter 6 is constituted by a so-called multiple inverter including a plurality of inverters and inverter transformers. Moreover, the function of the controlled rectifier 5 to control the output voltage thereof is utilized only for charging the battery 3 and not for the control of the output voltage of the uninterruptible power supply 100A. Thus, the output voltage of the uninterruptible power supply 100A is controlled by an inverter control circuit (not shown) by varying the output pulse width of the inverter 6. This requires that the inverter 6 functions to perform both DC to AC conversion and output voltage control, so that the control thereof becomes complicated. Further, the amount of higher harmonic components contained in the output voltage of the inverter 6 varies corresponding to the variation of the waveform thereof caused by the variation of the output pulse width of the inverter 6, so that the design of the AC filter 8 would be very difficult and the AC filter 8 larger in size would be necessitated.
As described above, a conventional uninterruptible power supply such as shown in FIG. 1 has been disadvantageous in that when an uninterruptible power supply is larger in capacity, an inverter, an inverter transformer and an AC filter are all rendered larger in size and higher in cost due to the complicated control technique.